1. Field of the Invention
The present invention relates to a sheet transporting or conveying apparatus, and more particularly to a transport path of a both-side unit adapted for use on an image forming apparatus such as a copying machine.
2. Related Background Art
This is already known an automatic two-side recording apparatus utilizing for example the electrostatic recording method, for use as a copying machine or a printer. The automatic two-side recording in such apparatus is generally achieved in one of following two structures.
The first is based on a S-shaped path provided in the copying machine P2 as shown in FIG. 1. Refering to FIG. 1, an original document is placed on a platen 152 provided on top of a main body 151 of the copying machine P2, and is scanned by an optical system 156 in said main body 151 to form a latent image on a photosensitive drum in an image forming unit 157. Said latent image is developed into a toner image therein. In said main body 151 there are provided plural cassettes 159, 160 and a manual insertion cassette 161, all containing sheets S. Also plural sheets S are stored in a stacker 163 of a cabinet 162, supporting the main body 151 of the copying machine. These sheets S are fed by a sheet feeder 165, consisting of rollers and separating rollers provided on each cassette.
The sheet S thus fed is subjected to the transfer of the toner image in a transfer station 166 of the image forming unit 157, then transported to a fixing unit 169 by a conveyor belt 167 for image fixation by heat and pressure, and is discharged onto a tray 172 a discharge unit 171 consisting of discharge rollers 170.
In case of two-side copying for forming images on both sides of a sheet S, or overlay copying for forming two or more images on a face of the sheet S, the sheet S bearing a toner image on a face thereof is not discharged from the apparatus but is guided, through a flapper 173, to a branch path 175. In case of two-side copying, the sheet S is guided through the S-shaped path 176, and is discharged onto an intermediate tray 179 by the rollers 177. In case of overlay copying, the sheet S is guided by deflection members 180, 181 provided in said S-shaped path 176, and discharged onto the intermediate tray 179 from the middle part of said S-shaped path 176. The sheets S stacked on the intermediate tray 179 are separated from the bottom and fed again through a re-feed path 182 to the image transfer station 166 for image transfer again, thereby forming a two-side copy or an overlay copy.
The second is based on a C-shaped path shown in FIG. 2. In case of two-side or overlay copying, the sheet S bearing the image on a first face thereof is guided through a flapper 173 to a branch path 175, in a similar manner as in the above-explained structure with S-shaped path.
In case of two-side copying, the sheets have to be stacked on an intermediate tray 183, with the face bearing the first image upwards. Consequently the sheet S is transported through a flapper 185 to a switchback path 186, then to a transport unit 187 above the intermediate tray 183, and is discharged thereon through a deflection member 189 or 190, or through discharge rollers 191 at the end of the transport unit 187. In case of overlay copying, the sheet S is guided through the flapper 185, not to the switchback path 186 but to the transport unit 187, and is discharged, from a predetermined position thereof, onto the intermediate tray 183.
For defining an appropriate discharge position depending on the length of the sheet S in the two-side or overlay copying, deflecting members 189, 190 are provided above the intermediate tray 183 thereby enabling to select plural discharge positions.
However, in the copying machine P2 with the S-shaped path explained above, the sheet transport path from the fixing station 169 to the re-feed path 182 inevitably becomes very long, thereby significantly increasing the dimension, particularly the height, of the copying machine. Also such long transport path increases the points of sheet removal in case of sheet jamming, thus increasing the trouble in jammed sheet removal.
Also in case of two-side copying, the sheets S discharged onto the intermediate tray 179 are aligned in the feeding direction, utilizing the inclination of the intermediate tray 179 and the movement of said sheets S by the weight thereof, so that a certain loss time is inevitable from the sheet discharge onto the intermediate tray 179 to the start of sheet re-feeding. Such loss time becomes more marked as the number of sheets S used for two-side copying decreases, and deteriorates the efficiency of the copying machine particularly in case of producing a single two-side copy.
On the other hand, in the copying machine P3 utilizing the C-shaped path explained above, the sheet S tends to be separated from the front-end stopper position at the re-feeding, particularly when a sheet of large size (such as A3 or B4 size) is discharged onto the intermediate tray 179. For this reason the stacking of the sheets S on the intermediate tray 183 is very unstable, as the sheets may be bent or curled at the front end in the course of sheet discharge, depending on the rigidity or curling tendency of the sheets.
Also because the deflecting members 189, 190 for small-sized sheets (such as A4 or B5 size) are protrudingly present above the intermediate tray 183, the distance between said tray 183 and the discharge path becomes inevitably small, rendering the removal of jammed sheet extremely difficult in case of transport failure in the discharge path or in the intermediate tray 183.